1. Field of the Invention
The present invention relates to a method in the electrolytic production of alkali metal chlorate which is obtained by crystallization and separation of the crystals from a solution recycled to the electrolysis process. A given proportion of the resulting crystals are separated and removed from the mother liquor after a time of contact with the mother liquor which is shorter than for the remaining crystals.
2. Description of the Prior Art
Alkali metal chlorate, in particular sodium chlorate, is used as a raw material in the production of chlorine dioxide which is an important bleaching chemical, especially for cellulose fibres. Sodium chlorate is produced by electrolysis of sodium chloride in aqueous solution. The solution circulates between an electrochemical reactor and reactor tanks in which further reactions take place. Part of the flow is conducted to a crystallizer in which sodium chlorate in solid form is recovered. The remaining part of the flow is recycled to the electrolysis reactor together with the mother liquor from the crystallizer and an equivalent amount of fresh sodium chloride. Crystallization takes place continuously or batchwise with direct, indirect or evaporative cooling. From the crystallizer, a slurry is taken from which the crystals are separated and washed. For easy handling, it is endeavoured to obtain relatively large crystals. Frequently, use is a made of a continuous crystallizer with classification, which implies that the mother liquor circulates in a manner such that the smallest crystals are taken along by a separate flow to which fresh solution is admixed, for recycling to the crystallizer.
The sodium chloride added is frequently contaminated by sulphate of which only a minor amount is removed from the system with the product. The sulphate is therefore accumulated in the electrolyte until the solubility product of sodium sulphate is exceeded. Depending upon local precipitations, however, difficulties arise in the form of excess voltages and increased energy costs already at lower sulphate contents in the electrolyte.
Sulphate can be removed from the system by precipitation with, for example, calcium or barium. However, this necessitates equipment for precipitation and filtration, simultaneously as the precipitate must be taken care of, for example dumped. In addition, calcium contents which are too high may cause precipitation on the cathodes, while high barium contents may cause irreversible reactions on the anodes.
U.S. Pat. No. 4,702,805 discloses a method in the electrolytic production of sodium chlorate, in which sulphate is removed from the system by causing a partial flow of the mother liquor from the chlorate crystallizer to undergo refrigeration, such that sodium chlorate crystals containing relatively large amounts of sulphate are formed and removed. The sulphate content in the electrolyte may be kept at such a low level that no difficulties arise. The idea is based on the fact that the solubility of sodium sulphate is substantially reduced at low temperatures, and this requires cooling to temperatures below 5.degree. C., preferably below -5.degree. C., and this again requires refrigerating equipment using refrigerants, such as freon or ammonia which normally are not available in chlorate production plants. This means that the investment and operating costs will be relatively high, and furthermore, the refrigerants are a hazard to the environment. Also, the mother liquor from the refrigeration has such a low temperature that it is not readily mixed with warmer electrolyte, so that layering or precipitation difficulties may arise. The chlorate crystallized by refrigeration is said to contain up to 11% by weight sodium sulphate, a content which is too high in many applications. The above technique is described also in C. Moser, "Control of Sulphate in the Production of Crystal Sodium Chlorate", Modern Chlor-Alkali Technology, Vol. 4, pp. 325-332.